Most vegetable seeds last at least 3 years, some more, some less. If the seeds were purchased commercially, they will have a date on their packet or container. Seeds should be used within 2 years of purchase, regardless of their variety. If you save your own seeds, this also applies.
To maximize the longevity of your stored seeds, there are a few things you can do. The first is to be sure they are properly labeled as to what they are and when they were prepared for storage (dried or preserved from the plant). Nothing is worse than looking at a box of seed packets and not knowing what they are or how old they are.
The primary concern with seed storage is keeping them dry. If the seeds become moist, they will germinate or rot, either of which is bad if you’re trying to preserve them. So proper storage is a must.
Cold Seed Storage
One of the more common ways to store seeds is to refrigerate them. Seeds are best preserved at 32-41 degrees Fahrenheit, which is the normal temperature inside a household refrigerator. These fridges are also usually moist, so you will need to protect the seeds from moisture by either sealing them in a container or using something to absorb the moisture (see below).
Many gardening households have jars of seeds in the back of their refrigerator to be used next spring.
Tips For Keeping Seeds Dry
There are several ways to keep seeds dry. One is to use glass containers with tight-fitting lids, as above, or zipper locking plastic storage bags to keep the moisture out. A dry storage room or shelf in a dry closet is another good location.
You can use several things to keep the seeds themselves dry in their container. Silica gel packets are sold in bulk at many craft supply stores (usually sold to dry flowers). These can be added to the container.
Adding powdered milk or diatomaceous earth (food grade) can be done as well. Powdered milk absorbs about twice its volume in water and can be added at a 1:1 ratio (by volume) with seeds. Diatomaceous earth absorbs up to 20x its volume in water and can be added in smaller amounts for the same results. Powdered milk is available in most grocery stores while diatomaceous earth can be purchased at any farm or feed and grain store and online. Both are low cost and usually sold by the pound.
They are best used by putting a quantity of seeds into a pouch made of porous cloth such as cheese cloth or burlap and placing it into a container filled with powdered milk or diatomaceous earth. This makes it easier to get the seeds out and to change out the powder when required. Powdered milk should be swapped every six months while diatomaceous earth can last a year or more (it doesn’t “go bad”).
DO NOT use reactive absorbents like baking soda as this can kill the seeds themselves because of the chemical reaction the soda will have on the seed hull.
More Seed Storage Tips
Having a good seed storage option is a must. Some enterprising seed savers keep theirs in a reverse humidor, which is a cigar humidor in which the humidifier has been reversed to pull moisture from the box. Again, keeping the seeds dry and relatively cool (under 80 degrees, closer to 40 being better) is top priority for storing them properly.